FIG. 7 is a block diagram showing a conventional AC servomotor control system. In this control system, a position deviation is obtained by subtracting a position feedback value detected by an encoder or the like from a position command, and a speed command is obtained by a position loop process in which the position deviation is multiplied by a position gain in term 1. A speed deviation is obtained by subtracting a speed feedback value from the speed command, and a speed loop process for proportional-plus-integral control is performed in term 2 to obtain a torque command (current command). Further, a current feedback value is subtracted from the torque command, then a current loop process is performed in term 3 to obtain voltage commands for individual phases, and PWM control or the like is performed to control an AC servomotor M.
To control a three-phase AC servomotor by the control system described above, there is known an alternating-current control method in which currents of three phases are separately controlled in the current loop. In this current control method, the torque command (current command) obtained by the speed loop process is multiplied by each of sine waves which are shifted by an electrical angle of 2.pi./3 for U, V and W phases, respectively, from a rotor position .theta. of the servomotor detected by an encoder or the like, to obtain the current command for respective phases. Then, current deviations are obtained by subtracting actual currents Iu, Iv and Iw of the three phases, which are detected by current detectors, from the obtained three current commands, and proportional-plus-integral (PI) control or the like is performed by current controllers for respective phases, to thereby output command voltages Eu, Ev and Ew for the phases to a power amplifier. The power amplifier performs PWM control by means of inverters etc., so that currents Iu, Iv and Iw of the individual phases are fed to the servomotor M to drive the same. In this way, a current loop is formed as an innermost minor loop in the position and speed loops, and this current loop controls the currents of three phases to be sent to the AC servomotor.
In the above method for controlling the currents of the three phases separately, since the frequency of each current command rises as the rotational speed of the motor increases to cause the gradual phase lag of the current, the reactive component of current increases to rise a problem that torque cannot be generated with good efficiency. Also, since the controlled variable is alternating current, even in a steady state in which the rotational speed and the load are constant, deviations such as a phase lag with respect to the command, attenuation of the amplitude, etc. occur, making it difficult to attain torque control comparable to that attainable with a direct-current motor.
As a solution to the above problems, a DQ control 30 method is known wherein the three-phase current is converted into a two-phase, i.e., d- and q-phase, direct-current coordinate system through a DQ conversion, and then the individual phases are controlled by direct-current components.
FIG. 8 illustrates a control system in which an AC servomotor is controlled through the DQ conversion. It is assumed that the d-phase current command is "0", and that the current command for q-phase is a torque command outputted from the speed loop. In a converter 9 for converting the three-phase current to a two-phase current, d- and q-phase currents Id and Iq are obtained by using actual currents of u-, v- and w-phases of the motor, and the phase of the rotor detected by a rotor position detector 7, and the currents thus obtained are subtracted from the command values of the respective phases, to obtain d- and q-phase current deviations. In current controllers 5d and 5q, the respective current deviations are subjected to proportional and integral control, to obtain d- and q-phase command voltages Vd and Vq, respectively. Another converter 8 for converting the two-phase voltage to a three-phase voltage, obtains u-, v- and w-phase command voltages Vu, Vv and Vw from the two-phase command voltages Vd and Vq, and outputs the obtained command voltages to a power amplifier 6, whereby currents Iu, Iv and Iw are fed to the respective phases of the servomotor by means of inverters etc. to control the servomotor M.
The conventional current control method described above has a problem that the current control system becomes unstable due to counter-electromotive force.
FIG. 9 is a diagram illustrating the conventional AC servomotor control system by separating it into d- and q-phase control systems. As illustrated, d- and q-phase controllers are provided with integral terms 11 and 12 (K1 is an integral gain) and proportional terms 13 and 14 (K2 is a proportional gain), respectively, and the motor is expressed as a combination of a resistance R and an inductance L. Also, the d and q phases are provided with interference terms 15 and 16, respectively. The d-phase controller controls a current component that does not contribute to a torque generated by the motor, and the q-phase controller controls a current component that contributes to the torque generated by the motor.
In FIG. 9, with the control method in which a d-phase current command Id* for the d-phase controller is set to zero while the torque command is applied to the q-phase controller as a q-phase current command Iq*, no reactive current flows in the d-phase direction, so that the current component that does not contribute to the driving of the motor can be removed, but in the q phase, a counter-electromotive force E (=.omega.e.multidot..PHI.) proportional to the rotational speed .omega.e of a motor is generated. FIG. 10 shows d- and q-phase voltage states during acceleration in the case where the d-phase current command Id* is set to zero. In FIG. 10, the circle represents a DC linkage voltage. The q-phase voltage R.multidot.Iq due to the resistance R of the q-phase winding shown in FIG. 9 is an active voltage for controlling the motor, while the voltage .omega.e.multidot.L.multidot.Iq generated in the d phase due to the interference term 15 is a reactive voltage that does not contribute to the driving of the motor. Symbol E denotes the counter-electromotive force. The terminal voltage of the motor is equal to the sum of the counter-electromotive force E and R.multidot.Iq. Control of the motor is possible when the terminal voltage is lower than or equal to the DC linkage voltage, and it is difficult to control the motor when the terminal voltage exceeds the DC linkage voltage.
FIG. 11 shows the d- and q-phase voltage states in which the counter-electromotive force E and the DC linkage voltage coincide with each other. In the case where the motor is accelerated to a high speed, the voltage for generating an acceleration current decreases due to an increase of the counter-electromotive force E. Therefore, the acceleration current decreases, and the counter-electromotive force finally coincides with the DC linkage voltage, thus terminating the acceleration. If the motor in this state is to be decelerated, a sufficient voltage is not available for generating a deceleration current, making it difficult to perform current control and possibly causing abnormal current flow.
In order to lower the terminal voltage of the motor in high-speed region, there is known a method in which the current phase is shifted in the d-phase direction when a heavy current flows in high-speed region. FIG. 12 shows the d- and q-phase voltage states when the current phase is shifted in the d-phase direction. In this case, the component Iqd in the d-phase direction of the q-phase current Iq flows in the d-phase direction, whereby the voltage .omega.e.multidot.L.multidot.Iqd generated in the q phase lowers the counter-electromotive force which appears in the terminal voltage. With this method, however, when the amount of current is small, the current in the d-phase direction is also small, thus rising a problem that the counter-electromotive force cannot be reduced sufficiently.